L-holds, l-pull-ups, rocking hollows, knees-to-elbows. Read a lot of Pavel's books (www.dragondoor.com), as he has many tension techniques for various exercises that will rip the hell out of your core. But yeah, L-holds and L-pull-ups will send you spiraling into hypertrophy quite nicely.

What is your take on the carry-over of high-rep cals like sit-ups, hollow and arch rocks, reverse hypers, glute-ham sit-ups, and back extensions to true tests of abdominal strength like front and back levers and planches?

I dropped them because I saw little carryover from low-intensity to high but a great deal of carryover from high to low. But I reflected on the fact that the adult gymnastics program I went to featured a healthy dose of high-rep abs, and that even Coach Burgener's WOD and Westside protocols have plenty of high-to-medium repetition ab and back cals. I have since added them back in, mostly doing bodyweight reverse hypers and v-ups.

We almost never do massive reps on stuff. We'll do Tabata V-ups and the like, but generally once we're above 15-20 reps on something we add resistance. eg hanging leg-lifts with ankle weights and the like.

BTW the ab/core componant of front/back levers and planches is the easy part :-P

The abdominal component of those exercises definetely IS easier; if I had some magic device that just took the need for lat strength out of the lever, it would be just a like a straight-body lift! And the planche!!!...